1. Field
The present invention relates to technology for improving light output of a gallium nitride-based vertical light emitting diode, and more particularly, to a gallium nitride-based vertical light emitting diode, which improves light output by providing a refractive index adjustment layer and an MgO pyramid structure on an n-type semiconductor layer, and a method for fabricating the same.
2. Discussion of the Background
A gallium nitride-based light emitting diode, which is used as a white light source, has high energy conversion efficiency, long lifespan and high light orientation, can be driven with a low voltage, requires no preheating time and complicated driving circuit, and is resistant against shock and vibration, making it possible to implement various types of high-class lighting system. Therefore, the gallium nitride-based light emitting diode is expected as a light source for solid-state lighting, which will replace the existing light sources, such as incandescent lamp, fluorescent lamp, and mercury lamp, in the near future. However, in order to use the gallium nitride-based light emitting diode as the white light source in replacement of the existing mercury lamp or fluorescent lamp, the gallium nitride-based light emitting diode must have excellent thermal stability and must be able to emit high-power light even at low power consumption.
A gallium nitride-based horizontal light emitting diode, which has been widely used as a white light source at present, has an advantage in that its fabricating cost is relatively low and its fabricating process is simple, but has a fundamental drawback in that it is inappropriate for use as a large-area high-power light source having a high applied current. A vertical light emitting diode is a device that overcomes the drawback of the horizontal light emitting diode and is easily applicable to a large-area high-power light emitting diode.
The vertical light emitting diode has many advantages as compared with the existing horizontal light emitting diode. Specifically, the vertical light emitting diode can obtain a very uniform current diffusion due to its small current diffusion resistance. Thus, the vertical light emitting diode can obtain a lower operating voltage and a high light output. Since the vertical light emitting diode can discharge heat through a metal or semiconductor substrate having excellent thermal conductivity, the lifespan of the device can be extended and the significantly improved high-power operation can be carried out.
In such a vertical light emitting diode, a maximum applied current is increased about 3-4 times or more as compared with the horizontal light emitting diode. Therefore, it is highly likely that the vertical light emitting diode will be used as the white light source for lighting. In practice, many companies have conducted extensive research and development for commercialization and performance improvement of the vertical light emitting diode, and some companies are selling related products.
On the other hand, in the fabricating of the gallium nitride-based vertical light emitting diode, a portion that can significantly improve light output of the device is an n-type semiconductor layer disposed in an upper portion of the device. However, when the n-type semiconductor layer is a smooth flat surface, a total reflection occurs in the interface of the atmosphere and the semiconductor layer due to a large difference in refractive index between the n-type semiconductor layer and the atmosphere (refractive index of the n-type semiconductor layer=about 2.5, refractive index of the atmosphere=1). A considerable part of light generated in an active layer cannot exit to the exterior. Consequently, high light output cannot be expected. Therefore, it is necessary to prevent the occurrence of the total reflection by artificially deforming the surface of the semiconductor layer, so as to exit light to the exterior with minimum loss.